This invention relates to a new and improved method and system for placing personnel, equipment, and material onto or in proximity to components of an energized electric transmission line system utilizing a generally ungrounded, aerial, overhead support platform and a suspension means composed of combinations of cable, nylon line and/or insulated components. In the electrical power generating utility industry, the servicing, repairing, maintaining, and construction of the transmission line system is done in either an energized state or de-energized state. The preferred method is in the energized state so that revenues are not lost. The electrical utility companies basically utilize either the live line or barehand technique when operating with an energized transmission line system to avoid damage or injury resulting from the flow of fault current.
Electricity is conveyed over long distances on conductors suspended by support structures that may be in excess of seventy-five (75) feet, such as metal lattice towers. Not only is access to the structures difficult and dangerous, but the presence of bundled energized conductors in three phases increases the danger of injury or damage from fault current discharge (arcing). Insulated components of the towers provide stand-off air gap distances between the high voltage conductors and the tower structure which is grounded. This air gap distance is required to prevent the charge on the conductor from arcing to the grounded support structure over the distance provided by the insulated component. This occurrence is referred to as a flash-over or arcing.
Certain arcing may be a relatively harmless discharge or transfer, such as between two differing ungrounded potentials, to equalize the potential. If the potentials are not grounded, there is no fault current. This discharge between ungrounded differing potentials will generally not occur unless they are very close together, usually a matter of inches. These types of discharges are generally not hazardous to personnel or ungrounded structures since minimal current flow occurs. This is the general principle behind barehand and live line maintenance techniques in use by the electrical utility industry today.
The more serious and lethal discharge is the fault discharge between differing potentials, one of which is grounded. The fact that one potential is grounded creates a greater stress on the insulating medium, whether it be the insulating component or an air gap, to convey the charge of the conductor to the grounded object. In order to prevent this type of flash-over, a greater distance must be maintained from the grounded differing potential, such as from the power line to any structure or object, whether it be a man, vehicle or the tower that is grounded. These distances have been determined and are utilized in the electrical utility industry as a standard stand-off distance in working with high power electrical lines. Typical values promulgated by OSHA and relied upon by the utilities industry are:
2.1-15 kv: 2'0" PA1 15.1-35 kv: 2'4" PA1 46.1-72.5 kv: 3'0" (high voltage) PA1 72.6-121 kv: 3'4" (high voltage) PA1 138-145 kv: 3'6" (high voltage) PA1 230-242 kv: 5' (high voltage) PA1 500-552 kv: 11' (extra high voltage) PA1 700-765 kv: 15' (extra high voltage)
As a grounded object approaches the energized component within a distance less than the flash-over stand-off values set forth above, the probability of a fault discharge or arcing to ground increases proportionately. The flash-over stand-off values set forth above have incorporated in them a certain safety factor and therefore adherence to them will generally insure that such a fault discharge will not occur. However, venturing inside those values will increase the potential for the fault discharge.
In live line procedures the repair, maintenance, inspection, and service of a transmission line system have been accomplished by ground crews utilizing apparatus mounted on generally ungrounded booms which are extended up to the structures or by men mounted on generally ungrounded bucket trucks and raised to the vicinity of the structures, and utilizing insulated apparatus to perform their functions. These methods are not only awkward but are only successful on those structures which may be reached by the available equipment. The greatest danger and obstacle arising in the utilization of these procedures is the fact that the booms and bucket trucks may become inadvertently grounded by structural deficiencies or failure of personnel to strictly adhere to safety procedures, and therefore the personnel operating them must adhere to the flash-over stand-off values set forth above when operating around energized structures. Utility companies have experienced fault flash-over as a result of ground crews inadvertently touching an energized component of the transmission line or allowing a grounded object to be positioned adjacent to an energized component within the stand-off value. If a fault flash-over occurs, a short circuit may result shutting down the entire power line and damaging the components of the power line requiring extensive repairs.
Regarding very tall structures, it is necessary that the maintenance personnel climb the structure to position themselves for repairs. Equipment and material must then be raised to them from the ground and again introduces the risk of a fault current discharge. Carts and other similiar pieces of equipment have been used to traverse the conductors, but transfer past support structures such as tower arms have proven difficult and time-consuming and raises the risk of a fault discharge from contact between energized and grounded objects. Inherent throughout these procedures is the danger of men moving about on the high structure in the vicinity of energized conductors.
As a result of the shut down from fault flash-overs, utility companies have experienced losses of revenues and have been required to make extensive repairs. For the foregoing reasons, and the fact that power lines are many times located in remote areas, access to them for repair, maintenance and inspection may be very difficult, dangerous, and time-consuming. This is especially true in developing Third World countries.
In the power generating utility industry the repair, maintenance, and inspection of energized transmission lines and their attendant structures usually requires that the system be shut down or de-energized to prevent the possibility of a fault discharge to any repair equipment or personnel placed in the vicinity of the energized components. Numerous attempts have been made in the industry to permit repair of the adjacent structures while allowing the line to remain energized thereby reducing the loss of revenues. This would require the use of an ungrounded platform so that fault current discharge could not occur. Various types of ground equipment, such as boom trucks, have been utilized but have been less than adequate because of the potential for fault discharge, structural default resulting in a grounding condition or negligence of personnel resulting in fault current discharge from energized components to the grounded or semi-grounded vehicle causing injury, death and damage.
A generally accepted method of repair, maintenance and inspection is that known as "barehand." This procedure involves the maintenance man generally attaching himself to the energized transmission line, raising his potential to that of the energized component. This "barehand procedure" maintenance technique has been developed and utilized in the electrical industry within the last seven to ten years. The procedures involve placing a lineman, a conductor cart, and/or equipment on the energized conductors themselves while the high-voltage transmission line is still in service. This technique has been developed and is being utilized primarily because of the high inherent costs of shutting or de-energizing a line in order to perform maintenance on it. This procedure facilitates such maintenance items as changing out insulators, effecting splice repairs of the conductor cable itself, installing repair sleeves, and repairing damage to the covering of the conductor such as armor rodding and spacer repair or replacements in bundled conductors. The inherent dangers associated with barehanding is primarily in the transferring of men and equipment to the energized component, i.e., from a grounded state to an ungrounded state. Various techniques that are being used are transferring or working from an insulated bucket truck, extension of an insulated ladder from the grounded tower to the conductors thereby providing an insulated transferring point for a man to the conductor, or through the use of block and tackle in swinging men and equipment from a grounded position to an ungrounded state to the conductor. All of these techniques are time-consuming and have exhibited potentially dangerous characteristics, either through equipment failure or accident, because of the involved procedures or failure to follow procedures. It is the intent of this invention to preclude these techniques of transfer when conducting barehand operations through the time-efficient and simple method as explained herein. Additionally, the absence of complicated procedures in effecting this technique or process further provide enhanced safety prospects. It is recognized by this inventor that the industry's needs in systems reliability are becoming extensive in the use of barehand techniques. Through this invention a considerable increase in time efficiencies and safety is effected when utilizing an overhead approach from an ungrounded position in space and the utilization of insulated links which embraces the use of simplified procedures and equipment to effect placement of men and materials on energized components.